This invention relates to passive detection systems for indicating the proximity of an Own station to any Other station that is equipped with a standard SSR (or equivalent ATCRBS) transponder, and to active detection systems employing an on-board interrogator at Own's station to detect any of said Other stations that are in proximity to Own station, and to combinations of such passive and active systems.
Many collision avoidance systems (CAS) using airborne transponder signals have been proposed. Some of these have been built and tested with varying degrees of success. The simplest systems merely receive Others' transponder replies, relying on the received signal strength to determine approximate range from Own. Other systems depend on two-way transmissions between Own and Other's transponder to provide range information. Said systems are subject to unacceptably high false alarm rates, particularly in dense aircraft environments, where reliable warnings are most needed. Still other systems involve airborne radio direction finding from Own to Others' transponders. Suitable direction finders have not been implemented, and are probably infeasible in the present state of the art.
Others have suggested the use of an on-board interrogator at Own's station to detect intruders. This, of course, works when there are no ground radars within range (providing the intruder's transponder is turned on in such an environment). It, however, suffers from two problems. First, when the density of equipped aircraft is high, the large total number of transmissions, which is related to the square of aircraft density, will degrade the performance of the ground ATC radar system, and possibly any other airborne SSR-based CAS. Second, when the density of other aircraft around such an interrogator is high, the replies from these aircraft will garble each other ("synchronous garble") and prevent decoding of the replies to determine relative altitude.
Other systems, using the time delay between reception at Own's station of an interrogation message from a particular SSR and reception of an Other reply to said interrogation, are described in numerous patents, for example the following:
U.S. Pat. No. 3,626,411 PA1 U.S. Pat. No. 3,735,408 PA1 U.S. Pat. No. 3,757,324 PA1 U.S. Pat. No. 3,858,210 PA1 U.S. Pat. No. 3,858,211 PA1 U.S. Pat. No. 3,875,570 PA1 U.S. Pat. No. 4,021,802 PA1 U.S. Pat. No. 4,027,307
These systems discriminate strongly against false alarms and provide various information about possible threats, such as range, bearing, differential altitude and identity. In general, the complexity of such a system is directly related to its capability. The technical feasibility of these systems has been demonstrated in a number of studies and tests. See, for example, the following reports: J. Vilcans et al., "Experimental BCAS Performance Results", U.S. Dep't. of Transportation, Report No. FAA-RD-78-53, dated July, 1978; and "Final Report For Feasibility Testing of the SSR Collision Avoidance System," dated June, 1976 and prepared under Contract No. F19628-75-C-0193 for the Electronic Systems Division of the United States Air Force Systems Command, Hanscom AFB, Bedford, MA 01731.
Research and development efforts on such systems have focused primarily on large systems suitable for commercial airlines which would three-dimensionally track transponder-equipped Other stations that are or might possibly become threats, visually display accurate bearing, range, and altitude information regarding such Other stations, including suitable alarms when a threat is detected, and also provide recommendations for evasive action such as turns, climbs and dives to avoid the threat. Using present-day technology, these large systems are presently expected to sell for over $60,000.00 each, which places them well above what most general aviation aircraft owners could afford or would be willing to spend.
The above-identified patents provide general background information which may assist those new to the art gain a fuller understanding of the operation of and advancements embodied in the present invention. Selected teachings from some of these patents are discussed below to assist those in the art to compare and contrast the present invention with earlier inventions and developments in the general field of proximity warning/collision avoidance systems.
U.S. Pat. No. 3,626,411 discusses, among other things, how the reception of at least two, and preferably three or more, successive 1090 MHz replies during an equal number of successive listen-in periods may be used to minimize the possibility of false proximity indications. It discloses a switch and pulse counter-based system for detecting such successive replies.
U.S. Pat. No. 3,735,408 discloses, among other things, a proximity warning system that indicates the presence of an intruder in a defined common azimuth sector that is wider by selectable amounts than the main SSR beam. In one embodiment of the system, the relative strength of the P2 pulses is monitored to determine when to expand or reduce the size of the widened azimuth sector. This allows the sector width to be deliberately reduced when the approximate distance between Own station and the SSR exceeds a predetermined amount. A 1090 MHz receiver disclosed therein includes a threshold device which initially raises the threshold of the receiver about 3 db for a 25 microsecond period during part of the listen-in period at Own station to discriminate against noise pulses or interference.
U.S. Pat. No. 3,875,570 (the '570 patent) discloses fairly complex proximity indicating systems having active and passive system portions which automatically adapt to changing SSR environments. Specifically, that patent discloses means for measuring the interrogation rate detected by Own station, and means for modifying the mode of operation of the proximity indicating system in dependence upon this rate to optomize system performance. As used therein, the term "interrogation rate" denotes the number of valid interrogations received from SSR ground stations per unit of time. Multiple modes of operation (two through five or more modes) based upon a classification of the SSR environment and/or upon a classification of the number of Other stations in the vicinity of Own station are also discussed. Further, it is disclosed that the azimuth listening sector of an SSR main beam may be widened or narrowed to vary the volume of monitored space. The patent also states the following. In a signal environment absent nearly all interrogations, the range of nearby aircraft may be obtained actively by the low-power omnidirectional transmission of interrogations. Such air-to-air interrogation may be on either the reply channel (1090 MHz) or the interrogation channel (1030 MHz) depending upon the sensing of the actual environment. Absent all ground interrogations the 1030 MHz channel may be used. The presence of many interrogations and replies requires the use of 1090 MHz interrogations (air-to-air) employing the proximity gated volumes of the threatening aircraft or its identity/altitude relationship to prevent undesired interrogations of non-threatening aircraft as well as undesired slant range replies.
The '570 patent also teaches that, in the case of a single identified intruder aircraft (a single Other station), the largest differential time of arrival (TOA or T) value from a group of TOA values determined in response to the interrogations of that aircraft by a multiplicity of SSR stations, say five or six, is the TOA value giving the most accurate indication of true slant range or separation between Own station and the intruder aircraft. Further, it discloses a means for storing such values and selecting the largest TOA.
The '570 patent also discusses Own station using a low power 1030 MHz interrogator with a very low interrogation rate to actively solicit transponder replies from possibly threatening aircraft in regions where SSR coverage is low or nonexistent, as determined by monitoring SSR interrogation rates.
U.S. Pat. No. 4,021,802 discusses a collision-predictive CAS that produces TAU data relating to differential azimuth, TOA, and/or altitude information from selected standard SSR interrogation and replies. In multiple SSR radar environments, the disclosed CAS operates in a manner that provides increased discrimination against false alarms. The patent discusses the identification of each SSR (and the data it produces) by its unique pulse repetition characteristic (PRC) and beam rotation period. Similarly, it discloses that all data obtained with regard to each particular transponder-equipped Other station may be identified and distinguished from that of different Other stations by the assigned identity reply code and/or any other suitably unique characteristic of Other's reply, such as its relative positional parameters. The patent discloses filtering means based upon the identification of and the individualized PRC selection of or lock to each SSR to identify and distinguish interrogations and replies associated with one SSR from those associated with another SSR. PRC is defined in the patent to include both uniform and staggered pulse repetition periods (PRP). The CAS disclosed shows a PRC selector as part of the filtering means used to associate a given reply containing identity or altitude information with a specific SSR. The patent also mentions that such a CAS may be entirely passive, or may be combined with other, active or semiactive, systems as a back-up or a false alarm filter.
U.S. Pat. No. 4,027,307 discloses a fairly complex collision avoidance/proximity warning system that determines both bearing and range, and employs among other things, a direction-finding antenna. It also discloses a passive collision avoidance system which stores multiple slant range values and averages them for a more accurate slant range value. Also disclosed is an active system for detecting the presence of an intruding aircraft and measuring its slant range and bearing by actively transmitting at 1030 MHz low power interrogations at a low rate such as 10 Hz. The patent further suggests that the active and passive systems may be used in combination, with the active system being used only when a passive range and bearing determination cannot be made.
Other development efforts, disclosed in U.S. Pat. Nos. 4,107,674 and 4,196,434, have also combined passive and active detection techniques in a single collision avoidance system. These two patents teach that while passive detection techniques may be used to detect the presence or absence of nearby transponder-equipped Other stations, active interrogations need to be used to obtain accurate range measurements. In both these patents, the passive detection systems disclosed incorporate not only time-based passive detection of Other stations, but also passive detection based upon the electric field strength of reply messages received from Other stations. The passively produced information is not used itself to determine whether a threat exists, but rather is used to control when and/or how the active system transmits 1030 MHz interrogations to effect an active range measurement between Own station and an Other station. Information from active range measurement is utilized either for determining whether Other station is a threat, or for collision-predictive tracking purposes.
U.S. Pat. No. 4,107,674 also teaches, among other things, that the interrogation transmission power and/or period between transmissions may be varied to reduce the amount of interference that active interrogations from Own station will cause to the existing SSR system. Specifically, the interrogations may be altered in accordance with the perceived distance between Own station and Other station as follows. If no Other station is detected, interrogations at 30 watts having a interval of 3 seconds between interrogations are transmitted from Own station. If an Other station is detected within a perceived distance of 10 nautical miles (NM) from Own station, interrogation power is increased to 300 watts while the interval between successive interrogations is increased to 12 seconds. As the detected Other station moves to within 4.5 NM as determined by active measurement, interrogation output power is decreased to 30 watts and the interval is linearly decreased from 12 seconds to 3 seconds. The patent also suggests altering the transmission power and the period based upon the difference in altitude between Own and Other, or upon the perceived approaching speed between Own and Other. One embodiment disclosed therein varies transmission power and period between the two aforementioned sets of values whenever the time-based passive detection value is less than 120 microseconds, or the received reply signal strength is over -60 dBm, or the number of interrogations in 12 seconds less than 100.
U.S. Pat. No. 4,196,434 discloses, among other things, an active/passive collision avoidance system wherein the number of interrogations per second from Own station and the output power is a function of position or location information obtained from previous active surveillance measurements. Specifically, the system disclosed therein uses two different interrogation power levels, and two different interrogation rates. The lower interrogation rate (2 sets of 10 interrogations each every 12 seconds) is used whenever at least one Other station is detected within 14 NM. It is also used if the number of interrogations received by Own station falls below 7 interrogations per second, which indicates that Own station is outside the coverage zone of SSR ground stations. The higher interrogation rate (1 set of 10 interrogations every 3 seconds) is used whenever an Other station is within 1.8 NM of Own or approaching faster than 43.5 m/sec, and has (or will shortly have) an altitude difference of less than 800 feet. In the words of the patent, "the interrogation period is selected as a function of the aircraft being an intruding or a threat aircraft." The higher interrogation power (300 watts) is the normal transmission power. The lower interrogation power (30 watts) is used in those instances where an Other station is within a 4 NM range of Own and (predicted) collision time is less than 40 seconds. The lower power level is said to reduce interference with Other stations and ground SSR stations, and make it possible for only the nearby aircraft to receive Own's interrogations.
The CAS disclosed in this patent also monitors the number of interrogations per second received by Own station to determine whether or not to increase the sensitivity of that passive detection equipment which monitors the field strength of received reply messages to determine whether any Other stations are within 14 NM of Own. If less than 20 interrogations per seconds are received (which is said to indicate roughly that less than 3 ground SSR stations are interrogating Own), the sensitivity level is increased from -65 dbm to -75 dBm within 12 seconds to provide greater detection capability.
A number of the foregoing patents, as well as other references in the art, set forth detailed facts relating to the characteristics of interrogation and reply messages in the SSR system, and the SSR system itself. While it is neither necessary nor desirable to review all such facts here, since most are common knowledge in the art, a few of them are mentioned below for convenient reference. Over 200,000 aircraft, including virtually all commerical transport aircraft and nearly all aircraft using major airports, are equipped with ATCRBS transponders. There are now approximately 3,000 SSR ground stations worldwide, and new stations are continuously being built. Each SSR ground station repeatedly transmits interrogations at a frequency of 1030 MHz on a continuously rotating beam. These beams, which conventionally are between 3 and 4 degrees wide and may occasionally be up to 6 to 7 degrees wide, sweep the sky in a clockwise direction. As the main beam of each SSR sweeps past an aircraft, it interrogates that aircraft's transponder with transmissions having a frequency of 1030 MHz, and each interrogation received initiates reply transmissions from the transponder at a frequency of 1090 MHz. Typically, a radar beam from an SSR site near an airport makes one full revolution every four seconds, while enroute longer range SSR radars typically revolve once every ten seconds, although some rotate as slowly as once every fifteen seconds. Effective beam range varies primarily upon transmission power, and commonly is between about 60 to 200 miles. Effective beam width varies, and depends not only on factors such as the design the radar antenna and distance of the aircraft to the radar, but also upon variations in both the transponder's SLS circuitry and the sensitivity of the transponder's 1030 MHz receiver. This last factor results from the fact that effective beam width is usually defined as the width of the space traversed by the beam axis while triggering a transponder.
Almost all SSRs are geographically distributed where possible to ensure effective radar coverage for use by aircraft controllers from different localities. Accordingly, the lines of sight between an aircraft at most any location and any two SSRs will not be congruent or superimposed on another. Such congruence does sometimes occur however, as when a short range SSR radar is positioned almost directly between an aircraft and a long range SSR radar. The characteristics of individual 1030 MHz interrogations and 1090 MHz replies, such as the pulses therein and their timing relationships, have often been described, as for example in ICAO Annex 10, and thus will not be repeated here. However, it is worthwhile to note that SSR ground stations continuously transmit interrogations at regular intervals between about 2 and 5 milliseconds. This interval is called the pulse repetition period (PRP). Its inverse value is called the pulse repetition frequency (PRF). Each SSR ground station within line of sight of an aircraft has a separate, unique PRF, and it also has a distinct beam rotation period (which for most short range SSRs near airports is typically somewhere about 4 seconds) to avoid sweeping the sky in synchronism with other nearby SSRs. The PRF's fall in the range of about 200 Hz to 400 Hz, separated by intervals of 4 or 5 Hz so that 40 to 50 separate PRF's are availabe for use by ground stations. Most often, the PRFs of SSRs are uniform, but occasionally so-called staggered PRFs are utilized, typically in denser SSR environments. This kind of pulse repetition pattern is composed of large steps in the spacings of the pulse train. 8-step staggers and 5-steps staggers are typical for this kind of SSR.
Typically, a main beam from an SSR, as it scans by an aircraft, repetitively interrogates the aircraft transponder between about 15 and 40 times. In response, the transponder generates a "beam burst" of repetitive replies that mimics the PRP of the interrogating beam. Not all interrogations produce replies however, and occasionally extraneous replies are produced by the transponder. These last two anomalies are due to such factors as ground clutter, multipaths, spurious signals, overlapping interrogations and the like. An interrogating beam may solicit up to thirty replies per beam burst, but a typical number of replies solicited has been reported to be about 18.
There is a current and compelling need for collision avoidance systems suitable for light aircraft such as those used in general aviation. Such systems have been designated TCAS-1. They must provide basically adequate collision threat warnings, but must also be simple and economically feasible for the owners of light aircraft. A paramount requirement is that they must minimize false alarms, while still assuring alarms in response to true threats.
In light of the foregoing problems and needs, it is the principal object of the present invention to provide a novel passive system and a novel active system, and to combine them in a novel manner which overcomes perceived limitations of using either a passive system or an active system by itself. Another very important object of the present invention is to provide a combined active/passive collision avoidance system which is economically affordable by the owners of light aircraft. Other objects of the present invention include: (1) combining the passive and active systems in a manner which allows the passive system to be continuously operational if desired; (2) providing an integrated passive/active system wherein selected elements and signals may if desired be jointly used by the passive system and the active system for increased reliability and reduced cost; (3) providing a simple and automatic means and/or method for sharing and for combining the information utilized in a passive system and in an active system; (4) providing a proximity warning system which does not require use of North pulses at SSR stations nor any other modification to SSR stations; (5) providing an effective proximity warning system for light aircraft which does not require determination of the course or bearing of either Own station or Other station; (6) providing a proximity warning system suitable for general aviation aircraft in any type of SSR environment including airspace covered by either zero, one, two, three, four or more SSRs; (7) providing an active/passive proximity warning system that automatically senses and adjusts itself to the various SSR environments it may operate in; (8) providing proximity warning system having interrogation capability which automatically adjusts itself in response to its environment in a manner which tends to minimize the risk of interfering with other ATC systems, whether ground based or airborne, including reducing the transmission rate and/or power of its active interrogations; and (9) providing a proximity warning system employing active interrogations having a randomized pulse repetition period (PRP) and/or randomized intervals between sets of transmissions to minimize interference with other ATC systems and to improve its own performance in higher traffic density environments.